This invention relates to the removal of contaminants from gas streams, and in particular this invention relates to the use of sorbents for removing sulfur from gas streams at low temperatures.
Vapor-phase fuel streams are valuable commodities. Natural gas consists of methane, carbon monoxide, hydrogen gas, and ethane. Liquified Petroleum Gases (LPG) are distillation products from both natural gas and oil-production processes, and are comprised of methane, ethane, propane, isobutane, butane and pentane. Other gases suitable for fuel gas use are derived from processes related to coal-gasification and oil shale-production. However, before such fuel streams are utilized, contaminants must be removed, particularly when advanced power generation systems are involved.
Sulfur-containing compounds (e.g. H2S and CS2) are the most typical contaminants in such fuel gas streams. Indeed, H2S concentrations may range from 100 grains/100 cubic feet in blue and carbureted water gas to several hundred grains per 100 cu. ft in coal and coke-oven gases. Refinery gases from sulfur crudes and natural gases from sulfur-bearing regions may contain H2S concentrations of several thousand grains per 100 cu. ft.
Removal of sulfur is necessary for both environmental reasons and also to protect the components (such as catalysts, fuel cells and turbines) of the power generation systems. Aside from power generation scenarios, certain chemical production processes also abhor sulfur in feed streams, for example, in natural gas-, ammonia-, oil-refining and petrochemical refining-processes. For example, approximately 14 percent of U.S. natural gas reserves contain sulfur in the form of hydrogen sulfide and at least 15 percent of the natural gas processed annually requires treatment to remove hydrogen sulfide.
In addition to hydrogen sulfide, sulfur-containing compounds such as mercaptans, organic sulfides, disulfides, thiophenes, thioesters, carbon oxysulfides, among others have to be removed from feed streams.
Some of the techniques for removing sulfur from feedstreams incorporates high temperature processes. For example, U.S. Pat. No. 4,089,809 assigned to the instant assignee, discloses a solid absorbent consisting of iron oxide supported on silica for removal of hydrogen sulfide from hot gaseous mixtures at temperatures between 538xc2x0 C. (1000xc2x0 F.) and 815xc2x0 C. (1500xc2x0 F.). A typical reaction process is as follows:
Fe2O3+3H2Sxe2x86x922FeS1.5+3H2Oxe2x80x83xe2x80x83Equation 1
Aside from the high temperature requirements of this process, the efficacy of silicon oxide sorbents for the absorption of hydrogen sulfide is further dictated by chemical equilibrium constraints, for example when water vapor in the untreated gas (i.e., on the left side of the equation) is above a certain level.
Another relatively high temperature sorbent, this one containing zinc oxide, was disclosed in U.S. Pat. No. 4,088,736, issued to Courty et al. This patent discloses a zinc oxide sorbent supported on silica and/or alumina. The temperature range during the absorption step is 200xc2x0 C. to 800xc2x0 C., and more particularly between 300xc2x0 C. and 650xc2x0 C.
Zinc ferrite sorbents and a calcium-pretreatment process are utilized in a feed stream desulfurization procedure disclosed in U.S. Pat. No. 4,769,045 to Grindley. The zinc ferrite sorbent is prepared by mixing and calcining equimolar amounts of zinc oxide and iron oxide. The temperature range during the absorption step is about 538xc2x0 C. to 649xc2x0 C. At temperatures below 677xc2x0 C., sulfur capture via calcium pretreatment is very minimal.
Zinc titanate sorbents have been developed to resist degradation at the high temperaturexe2x80x94and highly reducing coal gas-environments concomitant with hot-gas environs. The use of zinc titanate sorbents as high temperature desulfurization sorbents is disclosed in U.S. Pat. Nos. 4,313,820 and 4,725,415, both assigned to Phillips Petroleum Company. Absorption and olefin hydrogenation have a temperature range of about 149xc2x0 C. to 538xc2x0 C. and hydrodesulfurization in the range of about 205xc2x0 C. to 538xc2x0 C.
U.S. Pat. No. 4,977,123 to Flytzani-Stephanopolous et al., discloses a method of making mixed metal oxide sorbents suitable for use in fixed bed reactors. The mixed metal oxide absorbents are prepared using calcined powders of a desired composition as starting materials, adding water to form a paste, extruding the paste, and drying and heating the extruded paste to yield the desired extrudate strength. The oxides may be oxide mixtures of various metals such as for example, copper, iron, aluminum, zinc, titanium, and mixtures thereof. Inorganic binder materials such as bentonite clay may also be added. The disclosed absorption temperature is 650xc2x0 C.
The instant applicant, in U.S. Pat. No. 5,866,503, discloses the use of sorbent pellets for removing hydrogen sulfide in a coal gasification stream at temperatures at or above 260xc2x0 C. (500xc2x0 F.). Pellets are formed from a material reactive with hydrogen sulfide, a binder, and an inert material having a particle size substantially larger than the other components used to form the pellets. A diluent and a promoter may also be included during the formation process of the pellets.
Low temperature processes for removing sulfur from feed streams exist. Generally, however, these systems are low sulfur capacity processes. Some methods use wet processes operated within a liquid phase, typically an amine solution. These methods have the disadvantage of producing secondary waste streams such as contaminated waste water. Corrosion-, and solution loss-problems also exist with amine-type processing.
Activated carbon also is utilized in low-temperature sulfur removal processes. However, these systems have low capacities. And, the large carbon beds required are mostly non-regenerable, leading to secondary waste stream problems.
The Stretford process is another low-temperature approach for removing sulfur from feed streams. In the Stretford process, H2S gas is contacted with a scrubbing solution containing Vanadium in the +5 valence state and anthraquinone disulfonic acid (ADA) in a sodium carbonate solution at pH 9. The H2S is absorbed in an acid/base reaction and the resultant bi-sulfide ion is oxidized by the V+5 to produce elemental sulfur. V+5 is reduced to V+4 in this reaction but is regenerated using ADA and oxygen.
The Stretford process forms harmful substances and has very high capital costs. In addition, the process has not consistently achieved its design performance levels and has encountered many operating problems.
A need exists in the art for a solid sorbent to remove sulfur compounds at low temperatures. The sorbent should exhibit high sulfur capacity and be operable in the temperature range of between approximately 30xc2x0 C. and 200xc2x0 C. Furthermore, the sorbent should be relatively inexpensive to manufacture and maintain.
It is an object of the present invention to provide a sorbent for sulfur scavenging below 200xc2x0 C. which overcomes many of the disadvantages of the prior art.
Another object of the present invention is to provide a sorbent that has an increased sulfur absorbing capacity between 30xc2x0 C. and 200xc2x0 C. A feature of the invention is the combination of readily available sulfur-reactive materials with diluent and support materials to produce a porous sulfur-absorbing substrate. An advantage of the present invention is that less materials are required in the reactor bed resulting in minimization of the reactor bed size and prolonged use of the bed. Another advantage is that the material is useful in low-temperature production processes, thereby resulting in minimal costs.
Yet another object of the present invention is to provide a sorbent suitable for both fixed/moving and fluidized bed reactor applications. A feature of the invention is that the sorbents are comprised of metal-containing oxide which is reactive with hydrogen sulfide metal at the temperature range of 30 to 200xc2x0 C. An advantage of the sorbent is that it has excellent efficiency, and it can extract virtually all sulfur from the sulfur-containing feedstream so that near zero ppm levels of hydrogen sulfide concentrations in the feedstream are achieved.
Briefly, the invention provides material for absorbing sulfur, the material comprising a compound reactive with sulfur; an inert substance combined with the compound to create a mixture; and a binder to shape the mixture.
Also provided is a material for absorbing sulfur, the material comprising copper hydroxide; an inert material present at a weight percent of the material of approximately 7 to 12 percent; a binder material present at approximately 8 to 12 weight percent of the material; and a diluent material present at approximately 15 to 25 weight percent of the material.
Specifically, the invention provides a sorbent which is chemically and physically stable for use in sulfur removal processes at temperatures between 30xc2x0 C. and 200xc2x0 C., the sorbent comprising a material reactive with hydrogen sulfide, a binder unreactive with hydrogen sulfide, and inert diluents, wherein said material reactive with hydrogen sulfide is unreactive with all other components of said mixture.